Processes for forming a semiconductor layer, including compound semiconductor layers such as InP, InAs, GaAs, AlGaAs, InGaAs, GaInAsP and InGaAlAs layers, that comprise exposing a substrate in an evacuated chamber to an appropriate flux of molecules are well known. These processes will be generically referred to as molecular beam epitaxy (MBE) processes. See, for instance, Cho et al, Progress in Solid State Chemistry, Vol. 10, pp. 157-191 (1975). Various variants of the basic MBE process are also known. See, for instance, U.S. Pat. No. 4,330,360, and E. Veuhoff et al., Journal of Crystal Growth, Vol. 55, pp. 30-34. Among variants that are of interest herein are metal-organic MBE (MOMBE) and gas source MBE (also known as hydride source MBE). Compound semiconductor layers can also be produced by methods that involve contacting a substrate with an atmosphere that comprises precursor molecules. Exemplary of these processes is metal-organic chemical vapor deposition (MOCVD). These processes, together with MBE processes, will collectively be referred to as molecular growth processes.
It is general practice that during MBE deposition the substrate is maintained at an elevated temperature, since the quality of MBE-produced material frequently tends to improve with increasing substrate temperature T.sub.g, at least up to some upper limit that typically depends on the material. See, for instance, H. C. Casey et al., IEEE Journal of Quantum Electronics, QE11, p. 467 (1975). It is also well known that a III-V or other compound semiconductor material typically experiences a temperature-dependent loss of constituents when heated in a vacuum, the dependence of the loss rate of a given constituent on the temperature typically being exponential, and the loss rates for different constituents typically being different. In other molecular growth processes the substrate is also maintained at elevated temperature.
MBE deposition processes not only can be used to produce undoped but also doped compound semiconductor material. Doped material is typically produced by exposing a substrate to a flux of molecules that comprises, in addition to the constituents of the compound semiconductor, one or more dopant species. It is generally believed that the maximum effective carrier concentration that is attainable in device-grade III-V semiconductor material is determined by the solubility limit of the dopant species in the III-V material at the deposition temperature. Since typically the solubility increases with increasing temperature, this implies that the maximum possible effective carrier concentration increases with increasing substrate temperature during deposition of the doped III-V material.
Recently it has been reported that the maximum attainable electron concentration in Si-doped InGaAs increases with decreasing substrate temperature. T. Fujii et al., Electronics Letters, Vol. 22(4), p. 191 (1986). For instance, at substrate temperatures of 370.degree. C. and 420.degree. C. maximum electron concentrations of 6.1.multidot.10.sup.19 and 5.0.multidot.10.sup.19 cm.sup.-3, respectively, were achieved. The previously reported maximum value was 1.5.multidot.10.sup.19 cm.sup.-3, achieved at a substrate temperature of 500.degree. C., a conventional growth temperature.
Si is an important n-type dopant for III-V semiconductor materials. As is well known, it is an amphoteric dopant in these materials, that is to say, it locates in both lattice positions, the one normally occupied by the column III element or elements, and the one normally occupied by the column V element or elements. Since Si is an electron donor in III-V semiconductor material it however preferentially locates in the former lattice sites, and the effective electron concentration is determined by the difference between the Si occupancy of the two sites. This is exemplified by FIG. 2 of Fujii et al., which shows a much faster than linear decrease of the electron mobility as a function of effective electron concentration in Si-doped InGaAs, for concentrations above about 10.sup.19 cm.sup.-3, indicative of a total Si concentration that increases faster than the effective electron concentration.
Group II p-type dopants for III-V materials typically are non-amphoteric and thus can be expected to interact with the host lattice differently from amphoteric dopants. However, an ability to produce highly doped p-type device-grade III-V semiconductor material would be highly desirable For instance, it has recently been discovered that the inelastic scattering rate in such material can be much lower than previously thought, making possible an improved hot electron transitor. See U.S. patent application Ser. No. 241,279, entitled "Hot Electron Bipolar Transistor", filed Sept. 7, 1988 for A. F. J. Levi. See also U.S. patent application Ser. No. 250,790 entitled "Bipolar Hot Electron Transistor", filed Sept. 28, 1988 for Y. Chen et al., which discloses a very fast transistor comprising, inter alia, a thin p.sup.+ base layer (n=5.times.10.sup.19 cm.sup.-3). Both of these applications are co-assigned with this and are incorporated herein by reference.
Since the operating speed (and possibly other characteristics) of III-V transistors could be improved if it were possible to produce device-grade III-V semiconductor material that has a higher effective hole concentration than could heretofore be produced, it would be highly desirable to have available a method for producing such material. This application discloses such a method.
Throughout this application we will refer to some ternary and quaternary III-V materials in a manner that has become conventional in the art. For instance, any material of composition In.sub.x Ga.sub.1-x As will be referred to as InGaAs (to be read "indium gallium arsenide"), regardless of the value of x. The absence of numerical subscripts is thus not intended to imply the presence of one molar unit of each constituent element.